"Deterministic" is not the issue either, it's that small changes of the input will cause unknown changes in the output. You might theoretically achieve determinism and reproducibility for the exact same input (seeding the random number generators etc.), but the issue is that even if you formulate your request just a little differently, by changing punctuation for example, you'll get an entirely different output.

With compilers, the rules are clear, e.g. if you replace variable names with different ones, the program will still do the same thing. If you add spaces in places where whitespace doesn't matter, like around operators, the resulting behavior will still be the same. You change one function's definition, it doesn't impact another function's definition. (I'm sure you can nitpick this with some edge case, but that's not the point, it overwhelmingly can be relied upon in this way in day to day work.)